Wireless terminals

ABSTRACT

A wireless terminal for use in the transmitting and receiving frequency bands of a frequency duplex system comprises transmitting and receiving stages (Tx, Rx) and signal propagating means ( 22, 24, 26 ) coupled to the transmitting and receiving stages. The signal propagating means comprises a narrow band antenna structure ( 24 ), such as a Planar Inverted-F Antenna (PIFA), having sufficient bandwidth to cover the larger one of the transmitting and receiving frequency bands and a BAW receiving filter ( 26 ) and a BAW transmitting filter ( 22 ) coupled by respective feeds to the antenna structure ( 24 ). The filters ( 22, 26 ) enable the antenna structure to have a small volume and be reusable at different FDD frequencies.

TECHNICAL FIELD

The present invention relates to improvements in or relating to wirelessterminals, particularly, but not exclusively, to wireless terminalsoperating in accordance with protocols including frequency divisionduplex (FDD) systems, such as GSM, DCS and UMTS, having separatetransmit and receive frequency bands.

BACKGROUND ART

Typically cellular telephones have a common antenna for receiving andtransmitting signals within a relatively wide bandwidth. Various antennaarrangements are known in the art which have a wide enough bandwidth tocover both the transmitter and receiver frequencies used the FDD system.

U.S. Pat. No. 5,659,886 discloses in its preamble that in conventionalmobile units for digital radio communication, both the receiver andtransmitter are connected to a common receive/transmit antenna via atransmitting passband filter and a receiving passband filter. Thesefilters may be fabricated as dielectric filters or acoustic wavefilters. Since such components are difficult to fabricate as integratedcircuits and also they are relatively bulky, this patent specificationproposes that the transmitting bandpass filter be replaced by anisolator in order to reduce bulk. In the specific examples described,the common antenna comprises an external whip antenna. Isolators arethemselves are regarded as being inefficient devices because they candissipate power reflected from the antenna.

Wireless terminals, such as mobile phone handsets, sometimes have aninternal antenna, such as a Planar Inverted-F Antenna (PIFA) or similar.Such antennas are small (relative to a wavelength) and therefore, owingto the fundamental limits of small antennas, narrow band. However,cellular radio communication systems such as UMTS require a PIFA to havea fractional bandwidth of 13.3%. To achieve such a bandwidth from a PIFAfor example requires a considerable volume, there being a directrelationship between the bandwidth of an antenna and its volume, butsuch a volume is not readily available with the current trends towardssmall handsets. Hence, because of the limits referred to above, it isnot feasible to achieve efficient wide band radiation from smallantennas in present-day wireless terminals.

DISCLOSURE OF INVENTION

It is an object of the present invention to cover wanted frequency bandslying within a relatively wide bandwidth from a relatively small volumecommon receive/transmit antenna.

According to one aspect of the present invention there is provided awireless terminal for use in the transmitting and receiving frequencybands of a frequency duplex system, comprising transmitting andreceiving stages and signal propagating means coupled to thetransmitting and receiving stages, wherein the signal propagating meanscomprises an antenna structure having sufficient bandwidth to cover thelarger one of the transmitting and receiving frequency bands, areceiving filter and a transmitting filter coupled by respective feedsto the antenna structure.

According to a second aspect of the present invention there is provideda module for use in a wireless terminal operable in the transmitting andreceiving frequency bands of a frequency duplex system, comprisingsignal propagating means including an antenna structure havingsufficient bandwidth to cover the larger one of the transmitting andreceiving frequency bands, a receiving filter and a transmitting filtercoupled by respective feeds to the antenna structure and havingterminals for connection to the RF stages the wireless terminal.

The present invention is based on recognition of the fact that filterscan be used to make a narrow band antenna structure reusable atdifferent frequencies lying in a pass band bridging the transmitter andreceiver pass bands of a FDD system.

In an embodiment of the invention the antenna structure comprises aPIFA. The PIFA may include two differential slots which separate thePIFA into a central element and two outer elements which areinterconnected at one end. A free end of the central element isconnected to a ground plane and the free ends of the two outer elementsare connected respectively to the transmitting and receiving filters.

The filters may be solid state filters such as Bulk Acoustic Wave (BAW)and Surface Acoustic Wave (SAW) filters.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described by way of example, withreference to the accompanying drawings, wherein:

FIG. 1 is a block schematic diagram of an embodiment of a wirelessterminal made in accordance with the present invention,

FIG. 2 is a diagram of a circuit board having a PIFA and transmittingand receiving filters,

FIG. 3 is a diagram illustrating the radiating (or common) and balanced(or differential) modes of PIFA,

FIG. 4 is a diagram of the antenna structure connected respectively toBAW transmitter and receiver filters, and

FIG. 5 is the S₁₁ response of the antenna structure and BAW filters.

In the drawings the same reference numerals have been used to indicatecorresponding features.

MODES FOR CARRYING OUT THE INVENTION

Referring to FIG. 1, the transceiver comprises a transmitter section Txincluding a signal input terminal 10 coupled to an input signalprocessing stage (SPT) 12. The stage 12 is coupled to a modulator (MOD)14 which provides a modulated signal to a frequency up-convertercomprising a multiplier 16 to which a signal generator 18, such as afrequency synthesiser, is also connected. The frequency up-convertedsignal is coupled to a signal propagating structure 24 by way of a poweramplifier 20, a transmitter filter 22 and a matching/frequency tuningnetwork 23.

A receiver section Rx of the transceiver comprises a low noise amplifier28 coupled to the signal propagating structure 24, by way of amatching/frequency tuning network 25 and a receiver filter 26. An outputof the low noise amplifier 28 is coupled to a frequency down-convertercomprising a multiplier 30 and a signal generator 32, such as afrequency synthesiser. The frequency down-converted signal isdemodulated in a demodulator (DEMOD) 34 and its output is applied to asignal processing stage (SPR) 36 which provides an output signal on aterminal 38. The operation of the transceiver is controlled by aprocessor 40.

Referring to FIG. 2, a printed circuit board PCB has components (notshown) on one side and a ground plane GP on the reverse side. A PIFA 24is mounted on, or carried by, the PCB. The PIFA can be implemented inseveral alternative ways, for example as a preformed metal plate carriedby the PCB using posts of an insulating material, as a pre-etched pieceof printed circuit board carried by the PCB, as a block of insulatingmaterial having the PIFA formed by selectively etching a conductivelayer provided on the insulating material or by selectively printing aconductive layer on the insulating block or as an antenna on the cellphone case. For use at UMTS frequencies, the dimensions of the PIFA 24are length (dimension “a”) 30 mm, height (dimension “b”) 10 mm and depth(dimension “c”) 4 mm. These dimensions enable the PIFA 24 to havesufficient bandwidth to cover the larger of the FDD UMTS bands. Thebandwidth is substantially 3.1%. This is more than a factor of 4 lessthan the bandwidth required to cover the entire UMTS band (approximately13.3%). Nominally the PIFA 24 is resonant between the transmit andreceive bands.

The PIFA 24 has two differential slots 42, 44 extending lengthwise forpart of the distance from one edge to the other. The result is analogousto a comb having three prongs or elements PR1, PR2 and PR3interconnected at one of their ends and free at the other of their ends.The middle element PR2 is connected by a common shorting pin 46 to theground plane GP of the PCB. The element PR1 is coupled by a pin 48 tothe output of the transmitter filter 22 (FIG. 1) and the element PR3 iscoupled by a pin 50 to the input of the receiver filter 26 (FIG. 1).

The differential slots 42, 44 can also be used to tune the resonantfrequency of the antenna. Asymmetric slots, that is, slots of differentlengths and/or different shapes, will give different resonantfrequencies for the two feeds, viz. the pins 48, 50.

The differential slots are not essential but without them there is apotential problem of the inductance in the coupling to the filterfeeding the shorting pin 46. The slots increase the differential modereactance and facilitates isolation of the unused port, that is, thereceiver port in the transmit mode and visa-versa in the receive mode.This is illustrated in FIG. 3 in which the drawing shows on the left anembodiment of the PIFA 24 with the element PF2 shorted to ground and asignal source S1 coupled to the element PR1. An arrow 52 indicates thatthis feed arrangement constitutes a differential port. The PIFA 24connected in this way can be represented as being equivalent to thecombination of a radiating (or common) mode 24R and a balanced (ordifferential) mode 24B. In the radiating mode 24R, in-phase signalsources S2 and S3 are coupled to the elements PR1 and PR2, respectively,and the PIFA appears as a single one-piece antenna. In the case of thebalanced mode 24B, anti-phase sources S4 and S5 are coupled to theelements PR1 and PR2, respectively, so that current flows along PR1 toPR2 as shown by the arrows 54, 56 and a field exists across the slot 42.In this mode the differential mode reactance is increased and it iseasier to isolate the unused port by tuning the filter to present areflective termination, for example an open or short circuit to theantenna.

Referring to FIG. 4, the transmitter filter 22 comprises a 4-element,unbalanced, BAW ladder filter coupled to the antenna element PR1 by wayof the matching/frequency tuning network 23. This type of filter allowsan unbalanced input and output which is generally required for atransmitter. A source impedance represented by a 50 ohm impedance 60 iscoupled by a 2nH inductor 62 to the input of the filter 22. A 6nHinductor 64 couples an output of the filter 22 to the antenna elementPR1. The inductors 62 and 64 serve for tuning purposes and the value ofthe inductor 64 is optimised such that it also reduces the resonantfrequency of the PIFA 24 to that required for the transmitter frequencyband. Additionally, it is arranged such that it presents an approximateshort circuit in conjunction with the BAW filter's output staticcapacitance (not shown) at the receiver frequency.

The receiver filter 24 comprises a balanced, BAW lattice type of filterhaving a balanced input for connection to a 50 ohm source impedance 70which in the embodiment shown in FIG. 1 comprises the low noiseamplifier 28 and an unbalanced output coupled to the element PR3 of thePIFA 24. A series 1.5 nH inductor 72 and a shunt 2.4 pF capacitor 74 areprovided in the output circuit of the filter 24 and comprise thematching/frequency tuning network 25. The capacitor 74 increases theresonant frequency of the antenna and the inductor 72 ensures that thereceiver side is matched and that the combination of the transmitterfilter's static capacitance (not shown) and the external circuitrypresent an approximate short circuit to the antenna for the receiver.

FIG. 5 shows the S₁₁ response for the combined PIFA and filtercombination shown in FIG. 4 together with an idealised characteristic 84shown by a chain-dot line for a broadband antenna operating over theUMTS band of frequencies. The S₁₁ response comprises a transmittercharacteristic 80 shown by a full line and a receiver characteristic 82shown by a broken line. Referring to the transmitter characteristic 80the points referenced r1 and r2 and respectively indicate an attenuationof −18.428 dB at a frequency of 1.920 GHz and an attenuation of −6.282dB at a frequency of 1.980 GHz. In the case of the receivercharacteristic 82 the points referenced r3 and r4 respectively indicatean attenuation of −14.057 dB at a frequency of 2.110 GHz and anattenuation of −13.471 dB at a frequency of 2.170 GHz.

It is evident that an acceptable performance is achieved in both thetransmitter and receiver bands using an antenna that is too small tocover both bands simultaneously. In the combination shown in FIG. 4 thereceiver was optimised first and in consequence shows a betterperformance which is facilitated by the inherent better performance ofthe lattice filter 24. However it is believed that the transmitterperformance could be improved by further design iterations.

FIG. 5 confirms that the concept of utilising filters to make a compactantenna reusable at different frequency duplex frequencies is valid. Itis possible for similar results to be obtained with other types offilter besides BAW filters, such as SAW and ceramic filters.

In the present specification and claims the word “a” or “an” precedingan element does not exclude the presence of a plurality of suchelements. Further, the word “comprising” does not exclude the presenceof other elements or steps than those listed.

From reading the present disclosure, other modifications will beapparent to persons skilled in the art. Such modifications may involveother features which are already known in the design, manufacture anduse of wireless terminals and component parts therefor and which may beused instead of or in addition to features already described herein.

1. A wireless terminal for use in the transmitting and receivingfrequency bands of a frequency duplex system, comprising transmittingand receiving stages and signal propagating means coupled to thetransmitting and receiving stages, wherein the signal propagating meanscomprises an antenna structure having sufficient bandwidth to cover thelarger one of the transmitting and receiving frequency bands, areceiving filter and a transmitting filter coupled by respective feedsto the antenna structure.
 2. A terminal as claimed in claim 1,characterised in that the antenna structure comprises a PlanarInverted-F Antenna (PIFA).
 3. A terminal as claimed in claim 2,characterised in that the PIFA includes two differential slots.
 4. Aterminal as claimed in claim 3, characterised in that the twodifferential slots separate the PIFA into a central element and twoouter elements, the central and outer elements being interconnected, inthat a free end of the central element is connected to a ground planeand in that free ends of the two outer elements are connectedrespectively to the receiver and transmitter filters.
 5. A terminal asclaimed in claim 3 or 4, characterised in that the differential slotsare of substantially the same size and shape.
 6. A terminal as claimedin claim 3 or 4, characterised in that the differential slots areasymmetric.
 7. A terminal as claimed in any one of claims 1 to 6,characterised in that the transmitter and receiver filters are BulkAcoustic Wave (BAW) filters.
 8. A module for use in a wireless terminaloperable in the transmitting and receiving frequency bands of afrequency duplex system, comprising signal propagating means includingan antenna structure having sufficient bandwidth to cover the larger oneof the transmitting and receiving frequency bands, a receiving filterand a transmitting filter coupled by respective feeds to the antennastructure and having terminals for connection to the RF stages thewireless terminal.
 9. A module as claimed in claim 8, characterised inthat the antenna structure comprises a Planar Inverted-F Antenna (PIFA).10. A module as claimed in claim 9, characterised in that the PIFAincludes two differential slots.
 11. A module as claimed in claim 10,characterised in that the two differential slots separate the PIFA intoa central element and two outer elements, the central and outer elementsbeing interconnected, in that a free end of the central element isconnected to a ground plane and in that free ends of the two outerelements are connected respectively to the receiver and transmitterfilters.
 12. A module as claimed in any one of claims 8 to 11,characterised in that the transmitter and receiver filters are BulkAcoustic Wave (BAW) filters.